Flash memory devices are routinely used by computer users to store and retrieve programs, files, and/or other electronic data in nonvolatile memory. One type of flash memory device is a NAND Flash device. A NAND flash device typically utilizes a NAND Flash controller to write data to and read data from the NAND Flash device. The NAND Flash controller writes data to and reads data from the NAND Flash device page by page. Pages are typically grouped into blocks, where a block is the smallest erasable unit. For example, and without limitation, a typical memory device contains 2,112 bytes of memory per page and 64 pages of memory are contained in a block. The smallest entity that can be addressed within a page is a byte. A byte consists of eight bits, where a bit is the smallest allocable unit, logically representing a “1” or a “0.”
A typical 2 gigabit (Gb) NAND device is organized as 2,048 blocks. Each block contains 64 pages. Each page has 2,112 bytes total, comprised of a 2,048-byte data area and a 64-byte spare area. The spare area is typically used for Error Correction Code (ECC), wear-leveling information, and other software overhead functions. Typically, there is no limitation as to how the data area and the spare area are partitioned on the page. The NAND Flash control determines the data and spare area partition boundaries.
When the data within the pages of a block are no longer required, the block is erased. Erasure of the block typically takes 2 to 3 milliseconds (mS) during which time the device cannot be used for other operations. In comparison, programming a page of memory within a block usually takes 300 to 1200 microseconds (μS), and a read operation takes 20 to 60 μS depending on the memory array. One or more blocks of memory can be erased during a period of time when the controller would otherwise be idle as long as the controller has selected or marked one or more blocks of memory for erasure.
The memory in a NAND Flash device utilizes memory cells to store one or more bits of data. NAND Flash devices that store two or more bits of data in one cell are often referred to as multi-level cell (MLC) NAND flash devices. NAND Flash devices that store one bit of data in one cell are often referred to as single-level cell (SLC) NAND Flash devices. In a single-level cell, this single bit of data is represented in one of two states, known as bit states. One bit state logically represents a “1” and the other logically represents a “0.” In a multi-level cell that represents two bits of data, these two bits are represented in four bit states that logically represent “11,” “10,” “01”, and “00.” MLC technology results in obvious density advantages as shown in the example above in that two bits of data are represented in one memory cell instead of only one bit of data. The two bits of data are typically accessed using two pages within a block, a lower page and an upper page. If a typical SLC NAND Flash device has 64 pages per block the corresponding MLC NAND flash device has 128 pages per block. These pages are paired together, for example one bit within page zero is paired with one bit in the identical relative location in a different page. Data can then be stored in these bits on the different pages as further described below. One reason bits on different pages are paired together for programming operations involves error correction. If an error occurs during the programming process the error would manifest itself as a single bit error on two different pages which can be corrected, rather than two bit errors on a single page, which is more difficult to correct.
Typically, memory devices also move data from current blocks to unused blocks to avoid wearing out a particular memory block. This procedure is referred to as wear leveling. Wear leveling is used because, over time, the storage capacity of NAND Flash devices degrade as blocks are repeatedly used.
Currently, a number of rules exist to properly store and retrieve data from MLC NAND flash devices. For example, before first use a block of MLC NAND flash memory is left in an initial, erased state in which a “1” is stored in each memory location. Typically, specific bits of a page of a MLC NAND flash device are only permitted to be programmed once from its initial state to store data. Once data is stored in these specific bits of memory, these specific bits would typically need to be returned to their initial state, by an erasure operation on the block containing the specific bits, before new data could be stored in these bits. Once the erasure operation has been performed on all the bits contained in the block of memory, the specific bits of memory can be programmed again to store new information. The following rules are applicable to MLC NAND flash memory: 1) only a single bit within the memory cell can be programmed at a time since the two bits are represented as being in different pages; 2) a bit can be programmed from a higher logical state to a lower logical state (from a “1” to a “0”), but cannot be programmed from a lower logical state to a higher logical state (from a “0” to a “1”); and, 3) if a change in bit state occurs as a result of the programming step, the voltage of the new bit state must equal to, or exceed the voltage level of the previous bit state for the MLC NAND memory cell.
U.S. Pat. No. 6,982,905 entitled “METHOD AND APPARATUS FOR READING NAND FLASH MEMORY ARRAY” and U.S. Pat. No. 6,975,538 entitled “MEMORY BLOCK ERASING IN A FLASH MEMORY DEVICE” are commonly assigned to the assignee of the current application and are hereby incorporated, in their entirety, herein. Additionally, U.S. patent application Ser. No. 09/303,843 entitled “APPARATUS AND METHOD FOR PROGRAMMING VOLTAGE PROTECTION IN AN NON-VOLATILE MEMORY SYSTEM” publication no. US 2002/0027805; and Ser. No. 11/122,708 entitled “NAND FLASH MEMORY WITH IMPROVED READ AND VERIFICATION THRESHOLD UNIFORMITY” publication no. US 2005/0195651 are commonly assigned to the assignee of the current application and are hereby incorporated, in their entirety, herein.
A need exists for a faster way to target a block of memory contained in a MLC NAND flash memory cell for erasure. The targeted block of memory could then be erased during a time when the controller would otherwise be idle. Often which blocks are marked for erasure are stored by the controller, but in case the power is removed from the controller a method must exist to store this information in the NAND Flash memory device. This data is often stored within a location in the block that is to be erased. If old data is already in the block to be erased (hence the need for erasure) then at least one of the pages within the block must be programmed in a manner whereby the controller will know that the block is marked for erasure. If the power is removed from the controller, when power is reapplied the controller will know which blocks are marked for erasure. A need therefore exists for a method to program a page within a block more than once. In the first time, valid data will be programmed into the page containing valid data. In the second time, a mark must be programmed into the page to indicate that its block is ready for erasure.